Quickly erupted volcanic sections of the Steens Basalt, Columbia River Basalt Group: Secular variation, tectonic rotation, and the Steens Mountain reversal

The Steens Basalt, now considered part of the Columbia River Basalt Group (CRBG), contains the earliest eruptions of this magmatic episode. Lava flows of the Steens Basalt cover about 50,000 km² of the Oregon Plateau in sections up to 1000 m thick. The large number of continuously exposed, quickly erupted lava flows (some sections contain over 200 flows) allows for small loops in the magnetic field direction paths to be detected. For volcanic rocks, this detail and fidelity are rarely found outside of the Holocene and yield estimates of eruption durations at our four sections of ∼2.5 ka for 260 m at Pueblo Mountains, 0.5 to 1.5 ka for 190 m at Summit Springs, 1–3 ka for 170 m at North Mickey, and ∼3 ka for 160 m at Guano Rim. That only one reversal of the geomagnetic field occurred during the eruption of the Steens Basalt (the Steens reversal at approximately 16.6 Ma) is supported by comparing ⁴⁰Ar/³⁹Ar ages and magnetic polarities to the geomagnetic polarity timescale. At Summit Springs two ⁴⁰Ar/³⁹Ar ages from normal polarity flows (16.72 ± ± 0.29 Ma (16.61) and 16.92 ± ± 0.52 Ma (16.82); ± ± equals 2σ error) place their eruptions after the Steens reversal, while at Pueblo Mountains an ⁴⁰Ar/³⁹Ar age of 16.72 ± ± 0.21 Ma (16.61) from a reverse polarity flow places its eruption before the Steens reversal. Paleomagnetic field directions yielded 50 nontransitional directional-group poles which, combined with 26 from Steens Mountain, provide a paleomagnetic pole for the Oregon Plateau of 85.7°N, 318.4°E, K = 15.1, A₉₅ = 4.3. Comparison of this new pole with a reference pole derived from CRBG flows from eastern Washington and a synthetic reference pole for North America derived from global data implies relative clockwise rotation of the Oregon Plateau of 7.4 ± 5.0° or 14.5 ± 5.4°, respectively, probably due to northward decreasing extension of the basin and range.